Monomode optical fiber

ABSTRACT

The present invention concerns a monomode optical fiber comprising an optical core for guiding the majority of light waves, surrounded by an optical sheath, the difference between the maximum refraction coefficient of the core and that of the optical sheath being indicated as Δn and the radius of the core being indicated as a, characterized in that: 
     a ε 0.6a c  :1.1a c  !, a c  being provided, in μm, by the formula: ##EQU1## where k=2π/λ, λ being (in μm) the wavelength of the transmission, and where n g  is the coefficient of said optical sheath, equal within 10 -3  to that of pure silica, 
     Δn is greater than or equal to 0.01, 
     and in that, φ being the diameter of said optical sheath, in μm: φε φ min  :100!, φ min  being provided, in μm, by the formula: ##EQU2## where ##EQU3##

BACKGROUND OF THE INVENTION

The present invention concerns a monomode optical fiber.

An optical fiber is an optical waveguide consisting of a central part,referred to as an optical core, whose purpose is to guide the majorityof the light waves, surrounded by a peripheral part, referred to as anoptical sheath. In general, the optical sheath is surrounded by aprotective covering made of synthetic material (a resin, for example)with a thickness of approximately 50 μm.

The transmission wavelength currently chosen for optical fibers is 1.3or 1.55 μm. Indeed, it is at such wavelengths that it is possible toobtain a minimum attenuation in the transmission of light, inferior to0.4 dB/km.

Thus, within the scope of the present invention, the fibers consideredare intended to be used at a wavelength of 1.3 or 1.55 μm, the mostefficient for transmission.

Furthermore, it is well known that monomode optical fibers have a muchlarger bandwidth than multimode fibers, and it is for this reason thatcurrent and future developments in the field of optical fibers arefocused on monomode optical fibers.

As a result, the present invention is particularly related to monomodeoptical fibers intended to be used at a wavelength of 1.3 or 1.55 μm.

The monomode optical fibers currently used have a core diameter whichvaries from 8 to 10 μm; the diameter of their optical sheath is 125 μm,identical to that of multimode optical fibers. Because of the relativelysignificant diameter of the assembly made up of the optical core andsheath, the cost in silica to achieve the preform from which a standardmonomode optical fiber is drawn is high. To reduce this cost, it hasalready been suggested to reduce the diameter of the core toapproximately 2.5 to 3 μm, while maintaining the diameter of the sheathat 125 μm.

However, such a reduction in the diameter of the core does not lead to asufficient reduction in cost.

Furthermore, since the diameter of the optical sheath remains unchanged,the transmission capacity of a fiber-optical cable containing suchfibers is not increased.

SUMMARY OF THE INVENTION

One goal of the present invention is further to achieve a monomodeoptical fiber having a manufacturing cost inferior to that of knownmonomode optical fibers.

Another goal of the present invention is to decrease the dimensions ofthe monomode optical fibers so as to increase the capacity offiber-optical cables.

For this purpose, the present invention proposes a monomode opticalfiber comprising an optical core for guiding the majority of lightwaves, surrounded by an optical sheath, the difference between themaximum refraction coefficient of said core and that of said opticalsheath being indicated as Δn and the radius of said core being indicatedas a, characterized in that:

a ε 0.6a_(c) :1.1a_(c) !, a_(c) being provided, in μm, by the formula:##EQU4## where k=2π/λ, λ being (in μm) the wavelength of thetransmission, and where n_(g) is the coefficient of said optical sheath,equal within 3×10⁻³ to that of pure silica,

Δn is greater than or equal to 0.01,

and in that, φ being the diameter of said optical sheath, in μm: φεφ_(min) :100!, φ_(min) being provided, in μm, by the formula: ##EQU5##where ##EQU6##

By choosing the characteristic parameters of the optical fiber, i.e. a,Δn and φ, in the manner indicated above, it is possible to achieve amonomode optical fiber with transmission wavelengths approaching 1.3 or1.55 μm, which has a reduced sheath diameter with respect to the fibersof prior art, while displaying an attenuation and a sensitivity tomicrocurvatures comparable to those of standard fibers.

It can therefore be observed that, according to the invention, it is notnecessary for a monomode optical fiber to have an optical sheath with adiameter in the order of 125 μm. Such a diameter was chosen for theoriginal design of monomode fibers in order to obtain monomode andmultimode fibers with the same dimensions (for multimode fibers, whichhave a core diameter of up to 85 μm, the diameter of the optical sheathis close to 125 μm so as to obtain the guidance required whilepreserving the flexibility of the fiber).

The invention thus makes it possible to reduce, without adverselyaffecting the transmission performances, the overall dimensions of theoptical fiber and, as a result, to increase the capacity of thefiber-optical cables while reducing the cost of achieving opticalfibers.

The coefficient profile of an optical fiber according to the inventioncan be chosen from amongst the step-type profiles, trapezoid ortriangle, as long as the conditions for a, Δn and φ according to theinvention are observed.

Further characteristics and advantages of the present invention will beunderstood upon reading the description and examples which follow ofoptical fibers according to the invention, provided on an illustrativeand by no means limiting basis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of an optical fiber according to theinvention,

FIG. 2 shows the coefficient profile of the fiber of FIG. 1,

FIG. 3 shows a trapezoid-type coefficient profile,

FIG. 4 shows a triangle-type coefficient profile,

FIG. 5 shows φ_(min) as a function of Δn and a.

DETAILED DESCRIPTION OF THE INVENTION

In all these figures, the elements in common are indicated by the samereference numerals.

FIG. 1 thus shows an optical fiber 1 according to the inventioncomprising an optical core 2 made of a silica base material, forexample, doped with germanium to increase its refraction coefficientwith respect to that of pure silica, centrally located along thelongitudinal axis X of the fiber 1, surrounded by an optical sheath 3,made of substantially pure silica, i.e. with a refraction coefficientequal to that of pure silica or inferior to it by 10⁻³ or more (thecoefficient of the optical sheath 3 can vary between the periphery ofthe core 2 and the exterior of the fiber 1, while remaining inferior tothat of the core 2). The optical sheath 3 is surrounded by a protectivecovering 4 made of plastic material, such as a resin for example.

FIG. 2 shows the coefficient profile of the fiber of FIG. 1, i.e. itshows, as a function of the distance d from the axis X of the opticalfiber, the variations of the refraction coefficient n in the core 2 andin the optical sheath 3 of the optical fiber 1 within a cross-section ofthe fiber 1. The profile shown in FIG. 2 is of the type referred to asin steps: the maximum coefficient of the core 2 being indicated as n_(g)+Δn, the coefficient of the core 2 varies between n_(g) and n_(g) +Δn soas to give the curve a rectangular shape; the sheath 3 has asubstantially constant coefficient n_(g). As mentioned above, thecoefficient n_(g) typically neighbours that of pure silica, so that itis inferior to the latter (equal to 1.444) by a difference of 0.001 atmost.

FIGS. 3 and 4 show two other types of profiles which can be displayed bythe fiber 1, these being a trapezoid-type profile (the coefficient ofthe core 2 varies between n_(g) and n_(g) +Δn so as to give the curve atrapezoid shape, and the sheath 3 has a substantially constantcoefficient n_(g)) and a triangle-type profile (the coefficient of thecore 2 varies between n_(g) and n_(g) +Δn so as to give the curve atriangular shape, and the sheath 3 has a substantially constantcoefficient n_(g)), respectively.

In a general manner, the triangle and step-type profiles are considered,within the scope of the present invention, as particular cases of atrapezoid-type profile in which the ratio between the smaller base andthe larger base is equal to 1 or 0, respectively.

According to the invention, the outer diameter φ (in μm) of the opticalsheath can be chosen so that φε φ_(min) :100!, φ_(min) being provided,in μm, by the formula: ##EQU7## where ##EQU8## λ being (in μm) thewavelength of the transmission (1.3 or 1.55 μm), and a being the radiusof the core of the optical fiber.

When, according to the invention, φ is chosen within the intervalφ_(min), 100!, i.e. so as to reduce losses due to curvatures ormicrocurvatures, Δn being chosen superior to 0.01 and a (in μm) withinthe interval a_(c) :1.1a_(c) ! (a_(c) being provided, in μm, by theformula: ##EQU9## so as to maintain within the optical fiber anattenuation inferior to 0.4 dB/km and preserve a cut-off wavelengthstrictly inferior to the wavelength of the transmission (1.3 or 1.55μm), it is possible to bring the diameter of the optical sheath wellbelow 125 μm (conventional value for standard monomode fibers), whichmakes it possible to achieve significant savings in the manufacturingcost of optical fibers, as well as to increase the capacity of thefiber-optical cables.

For example, when φ is chosen equal to 100 μm, with all the otherelements unchanged with respect to those in a standard monomode opticalfiber, the reduction in the dimensions of the optical fiber obtained isclose to 33%; when φ is chosen equal to 80 μm, with all the otherelements unchanged with respect to those in a standard monomode opticalfiber, the reduction in the dimensions of the optical fiber obtained isclose to 60%. As a first approximation, the reduction in themanufacturing cost of the fiber is substantially equal to the reductionin cross-section, i.e. to the reduction in dimensions.

FIG. 5 shows a monomode fiber intended to operate at 1.55 μm, severalcurves providing the minimum diameter of the optical sheath φ_(min) fordifferent values of the ratio a/a_(c), as well as for different valuesof Δn.

The curve 51 corresponds to the case where Δn is equal to 0.01; it canbe seen that φ_(min) can be chosen from between 75 μm and 100 μm,according to a/a_(c).

The curve 52 corresponds to the case where Δn is equal to 0.015; it canbe seen that φ_(min) can be chosen from between 62.5 μm and 100 μm,according to a/a_(c).

The curve 53 corresponds to the case where Δn is equal to 0.012; it canbe seen that φ_(min) can be chosen from between 50 μm and 75 μm,according to a/a_(c).

The criteria according to the invention with respect to a, Δn and φapply in a general manner to all optical fibers having a trapezoid-typeprofile (including the optical fibers having a triangle or step-typeprofile).

As for the synthetic protective coating 4, its thickness is in the orderof 50 μm.

A few specific examples of optical fiber structures in conformity withthe present invention are provided hereafter. Examples 1 to 8 concernfibers intended to operate at 1.55 μm, and example 9 concerns fibersintended to operate at 1.3 μm.

In all these examples, the thickness of the coating 4 is 50 μm.

All the fibers obtained according to the invention and displaying thecharacteristics indicated in the following examples have an attenuationinferior to 0.4 dB/km and a sensitivity to microbubbles at leastcomparable to that of a standard fiber with an optical sheath diameterof 125 μm, a core diameter comprised between 8 and 10 μm, and aprotective coating thickness equal to 50 μm.

EXAMPLE 1

In this example, an optical fiber according to the invention with astep-type profile has the following characteristics (for 1.55 μm):

Δn=0.01

a=2.43 μm

φ=100 μm

EXAMPLE 2

In this example, an optical fiber according to the invention with astep-type profile has the following characteristics (for 1.55 μm) :

Δn=0.01

a=2.60 μm

φ=90 μm

EXAMPLE 3

In this example, an optical fiber according to the invention with astep-type profile has the following characteristics (for 1.55 μm):

Δn=0.016

a=1.7 μm

φ=100 μm

EXAMPLE 4

In this example, an optical fiber according to the invention with astep-type profile has the following characteristics (for 1.55 μm) :

Δn=0.015

a=1.98 μm

φ=80 μm

EXAMPLE 5

In this example, an optical fiber according to the invention with astep-type profile has the following characteristics (for 1.55 μm) :

Δn=0.015

a=2.26 μm

φ=70 μm

EXAMPLE 6

In this example, an optical fiber according to the invention with astep-type profile has the following characteristics (for 1.55 μm):

Δn=0.02

a=1.47 μm

φ=80 μm

EXAMPLE 7

In this example, an optical fiber according to the invention with astep-type profile has the following characteristics (for 1.55 μm) :

Δn=0.02

a=2.45 μm

φ=70 μm

EXAMPLE 8

In this example, an optical fiber according to the invention with astep-type profile has the following characteristics (for 1.55 μm) :

Δn=0.02

a=2.45 μm

φ=60 μm

EXAMPLE 9

In this example, an optical fiber according to the invention with astep-type profile has the following characteristics (for 1.3 μm) :

Δn=0.01

a=2.038 μm

φ=96 μm

EXAMPLE 10

In this example, an optical fiber according to the invention with astep-type profile has the following characteristics (for 1.3 μm) :

Δn=0.015

a=1.426 μm

φ=96 μm

EXAMPLE 11

In this example, an optical fiber according to the invention with astep-type profile has the following characteristics (for 1.3 μm):

Δn=0.02

a=1.644 μm

φ=57.5 μm

Obviously, the present invention is not limited to the embodimentsdescribed above.

In particular, the protective coating made of a synthetic material canconsist of one or more layers, and its thickness does not necessarilyneighbour 50 μm; this thickness is chosen according to the mechanicalProperties sought for the optical fiber, as well its sensitivity tocurvatures and microcurvatures.

Furthermore, the invention also applies to optical fibers whose profileis of trapezoid or triangle-type. The reference parameters (a, Δn and φ)for a fiber of this type can be transposed through computation to thereference parameters of a fiber with a step-type profile, and viceversa, as there is a continuous equivalence relation between theseparameters. In this case, the step-type profile is said to be equivalentto a triangle or trapezoid-type profile. Thus, the examples of profilesprovided above can be transposed in a simple manner, by using thisequivalence relation, to triangle or trapezoid-type profiles, i.e. thereference parameters of the triangle or trapezoid-type profiles can bederived in a direct manner.

Finally, any means can be replaced by an equivalent means withoutdeparting from the scope of the invention.

We claim:
 1. A monomode optical fiber comprising an optical core forguiding light waves, surrounded by an optical sheath, the differencebetween the maximum refraction coefficient of said core and that of saidoptical sheath being indicated as Δn and the radius of said core beingindicated as a, characterised in that:a ε 0.6a_(c) :1.1a_(c) !, a_(c)being provided, in μm, by the formula: ##EQU10## where k=2π/λ, λ being,in μm, the wavelength of the transmission, and where n_(g) is therefraction coefficient of said optical sheath, equal within 10⁻³ to thatof pure silica, Δn is greater than or equal to 0.01, and in that, φbeing the diameter of said optical sheath, in μm: φε φ_(min) :100!,φ_(min) being provided, in μm, by the formula: ##EQU11## where ##EQU12##2. An optical fiber according to claim 1, characterized in that itdisplays a step, trapezoid or triangle-type profile.
 3. An optical fiberaccording to claim 1, characterized in that said optical sheath issurrounded by a protective coating made of a synthetic material.
 4. Anoptical fiber according to claim 3, characterized in that saidprotective coating has a thickness which neighbours 50 μm.